Get one of the
cheap Chinese 8kw units, like the Vevor. For 130€ they
work a charm.
I'm from
Germany and it's a bit sad to see that Webasto and Eberspächer
lost a lot of that business, but their patents on this are long gone and their units costs 20 times as much.
Their controls are probably a bit better, but at that
price tag...
The boxed units have the advantage that you can easily do
maintenance if it's ever needed.
Add an inline
diesel filter.
If you invest another 50€ into a spare package (Combo of circuit board+control boat+remote &
pump & thermo sensor) your going to be fine even in the rare event something stops working.